1. Field of the Invention
The present invention relates to a high-melting-point conductive oxide, to a method of manufacturing the oxide, and to a high-temperature electrically conductive material used to form high-temperature heat-generating materials, such as heating elements for generating high temperatures in electric furnaces and the like, electrode material capable of being used at high temperatures, high-temperature light-emitting material including filament material for lamps and the like, and high-temperature thermocouple material and the like, utilizing heat produced when a current is passed through the oxide in an atmosphere containing oxygen.
2. Description of the Prior Art
Electric furnaces that utilize resistance-heating employ a heating element material selected for that purpose. In air or an atmosphere including an oxygen, silicon carbide (SiC) heating elements can be used to obtain temperatures in the order of 1600xc2x0 C., and molybdenum disilicide (MoSi2) heating elements can be used to obtain temperatures up to around 1700xc2x0 C. Platinum (Pt) or platinum-rhodium (Ptxe2x80x94Rh) metal wire has a melting point of 1769xc2x0 C., and can be used for electric furnaces that go up to 1500xc2x0 C. Including the temperature dependency, the electrical resistivity of these heating elements is more or less metal-like. Heating elements of lanthanum-chromite, an oxide of lanthanum and chromium, has a semiconductor-like temperature dependency, but because of the material""s relatively low electrical resistance, can be used to form general-purpose electric furnaces capable of obtaining temperatures of up to 1900xc2x0 C.
To obtain higher temperatures with resistance-heating methods, zirconia (ZrO2) or thoria (ThO2) having a melting point or decomposition temperature of 2000xc2x0 C. or more is used. Since these oxides are semiconductors, current flow can only occur at high temperatures of at least 1000xc2x0 C. That is, since they have to be used in conjunction with other heating elements with metal-like resistivity, general-purpose electric furnaces that use these oxides as heating elements have not yet been realized. In hydrogen or an inert gas or other reducing atmosphere, the high-melting-point metals of molybdenum (Mo), tantalum (Ta) and tungsten (W) can be used to obtain high temperatures of up to 2000, 2100 and 2500xc2x0 C., respectively. In the presence of very small amounts of oxygen (10xe2x88x925 Torr or more), these heating elements readily form oxides, increasing their electrical resistance. For this reason, they cannot be used in an oxygen atmosphere.
Noble metals such as ruthenium (Ru), iridium (Ir) and rhodium (Rh) have high melting points (ruthenium: 2250xc2x0 C.; iridium: 2457xc2x0 C.; rhodium: 1963xc2x0 C.). However, in an oxidizing atmosphere, at high temperatures these metals readily vaporize or oxidize. Moreover, the fact that they are noble metals mean that they are very expensive, costing four or five times more than gold, and they are not easy to machine, all of which makes it very difficult in practice to use them as heating element materials.
JP-A-HEI 6-223960 discloses high-frequency heating elements formed of oxides of ruthenium, iridium, rhodium or other elements. The invention of this disclosure has as its object the use of microwave absorption for heating applications such as microwave ovens and the like, and is characterized in that when the above materials are used to form heating elements for high-frequency heating, the materials exhibit good temperature elevation characteristics. The materials are intended for use at around 600xc2x0 C. to 870xc2x0 C., which is not the same as materials intended to be used at temperatures in the order of 2000xc2x0 C.
Previously, resistance-heating to temperatures of 2000xc2x0 C. or higher has only been possible using heating elements of tungsten or other such high-melting-point metal in a reducing atmosphere of hydrogen or an inert gas or the like. There have not been any suitable materials, that could be used in an oxidizing atmosphere, having a melting point in the order of 2000xc2x0 C. and metal-like electrical resistivity. Therefore, there has not been a practical electric furnace capable of achieving a temperature of 1700xc2x0 C. or higher. In other words, there has been no method of generating a high temperature of at least 1700xc2x0 C. utilizing resistance-heating in an oxidizing atmosphere. Similarly, there has no thermocouple material to measure a temperature of 1700xc2x0 C. or more. However, if there were a material having a metal-like low electrical resistance and a melting point of around 2000xc2x0 C. in an oxidizing atmosphere, it would be possible to provide a method of resistance-heating at a previously unattainable temperature range, and it would also be possible to use a thermocouple to measure such high temperatures.
An object of the present invention is therefore to provide a high-temperature electrically conductive material that can be used for heating elements, electrode material, thermocouple material, light-emitting material and other such materials in an electric furnace that can be used in an oxidizing atmosphere at a high temperature of 1700xc2x0 C. or more.
Another object of the present invention is to provide a high-melting-point electrically conductive oxide constituting the high-temperature electrically conductive material, and a method of manufacturing the same.
To attain the above object, the present invention provides a high-melting-point conductive oxide that is a Srxe2x80x94Ru oxide comprising a mixture of a powdered Sr compound and Ru compound or Ru metal that is sintered at 900xc2x0 C. to 1300xc2x0 C. in an atmosphere containing oxygen to form a sintered body that is pulverized to a powder, shaped and again sintered at 1000xc2x0 C. to 1500xc2x0 C. in an atmosphere containing oxygen.
The above object is also attained by a high-melting-point conductive oxide that is a Srxe2x80x94Ru oxide comprising a mixture of powdered SrCO3 and RuO2 that is sintered at 900xc2x0 C. to 1300xc2x0 C. in air to form a sintered body that is pulverized to a powder, shaped and again sintered at 1000xc2x0 C. to 1500xc2x0 C. in air.
The above object is also attained by a high-melting-point conductive oxide that is a Srxe2x80x94Ru oxide single-crystal comprising a Srxe2x80x94Ru oxide obtained as described above that is melted by concentrated infrared radiation or the like and recrystallized.
The present invention also includes the oxide and oxide single-crystal in which a molar ratio between the SrCO3 and the RuO2 is 2:1.
The above object is also attained by a method of manufacturing the high-melting-point conductive oxide, the method comprising mixing together powdered SrCO3 and RuO2 to form a mixture, sintering the mixture in air at 900xc2x0 C. to 1300xc2x0 C. to form a sintered body, pulverizing the sintered body to a powder, shaping the powder, and sintering the shaped powder in air at 1000xc2x0 C. to 1500xc2x0 C.
The present invention also includes a method of manufacturing the Srxe2x80x94Ru oxide single-crystal by using focused infrared radiation or the like to melt the Srxe2x80x94Ru oxide obtained as described above, and recrystallizing the melted oxide.
The present invention also provides high-temperature heating elements, high-temperature electrode material, light-emitting material and high-temperature thermocouple material comprised of the oxide or single-crystal for use in an atmosphere containing oxygen.
As described in the foregoing, as a material for use as heating elements, electrode material, light-emitting material and thermocouple material, of all the conductive oxides, there is used an oxide or single-crystal containing Sr and Ru, which in an atmosphere containing oxygen has the highest melting point and exhibits metal-like electrical resistance up to temperatures close to the highest melting point. The result is material properties that do not undergo change even at 2000xc2x0 C. and can effect, for the first time, formation of oxidizing atmospheres at 2000xc2x0 C. and measurement of temperatures in oxidizing atmospheres at 2000xc2x0 C.
The above and other objects, further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.